What force binds stars together in both open and globular clusters?

The force that binds stars together in both open and globular clusters is gravity. Gravity is the fundamental force that acts on all objects with mass, pulling them toward one another. In the context of star clusters, every star exerts a gravitational pull on every other star in the cluster. This mutual attraction is what allows stars to remain bound in a group despite their individual motion through space.

In open clusters, which are typically loosely bound and contain just a few hundred stars, gravity is strong enough to keep the stars together for millions of years, but over time, they can disperse due to gravitational interactions with other clusters or the surrounding galactic environment. In globular clusters, which are densely packed with hundreds of thousands to millions of stars, gravity plays an even more crucial role. The intense gravitational forces help to maintain the cluster's structure, allowing it to exist for billions of years.

Other forces listed, such as electromagnetic force and nuclear force, play significant roles in atomic and subatomic interactions but do not directly affect the large-scale structure of star clusters. Friction, while relevant in many physical contexts, does not have any significant role in the dynamics of stars within a cluster, as space is a near-vacuum where such interactions are negligible.